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Liquid Crystal Photonic Devices

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In this presentation, I will adress the liquid crystal photonic devices. For this purpose, we studied mainly two different kinds of photonic effect, i.e., (1) lasing and (2) electro-tunable optical diode effect in a sophisticated structure containing an anisotropic nematic layer and cholesteric liquid crystal films. Section 1 described the overview of the photonic effect particularly in cholesteric liquid crystals. The following 5 sections are devoted to describe major topics studied in this thesis. In section 2, we demonstrated the characteristic of new type of defect mode lasing from a dye-doped 2-micrometre thick anisotropic nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap materials. The nematic layer acts as an anisotropic defect layer, the anisotropy of which brings about the following remarkable optical characteristics; (1) reflectance in the photonic band gap region exceeds 50 % due to the retardation effect, being unpredictable from a single CLC film, (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge, and (3) the lasing emission both from the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized. We also demonstrated the electro-tunable lasing of the anisotropic NLC layer introduced between photopolymerized CLC films. Field-induced tunable lasing could be achieved by increasing applied voltage due to the decrease of optical path length of an anisotropic NLC layer caused by the reorientation of NLC molecules. In section 3, we found the optical characteristics of a thick anisotropic NLC layer introduced between PCLC layers such as anisotropic mode spacing. The transmittance spectra of a 100-micrometre-thick anisotropic NLC layer displayed several dips within PBG , that is different form that of a 100-micrometre-thick isotropic layer. We also found that the LCP (opposite to the helix handedness) component shows a very large modulation within the PBG . Multi-mode lasing without any particular polarizations occurs within the PBG at the wavelength corresponding to the transmittance maxima due to the new structure. In section 4, we introduce experimental results toward CW lasing, which are desirable in view of practical application, using defect mode or Fabry-Perot cavity realized by the structures that include both a spacing layer defect and a phase effect. In section 5, we studied another type of defect modes by introducing different optical pitch and handedness of CLC between two PCLC layers, giving rise to additional resonant modes by the results of transmittance, polarization of lasing, and threshold. To confirm that this is a new type of defect mode, we introduce 3 types of cell configuration and compare them; i.e., (a) R-helical PCLC / polymeric dye-doped L-helical CLC / R-helical PCLC , (b) R-helical PCLC / polymeric dye-doped L-helical CLC , and© polymeric dye-doped L-helical CLC . Introducing different optical pitch and handedness of CLC between two PCLC layers produces high-Q laser cavities, and it shows efficient lasing emission with low threshold value. The lasing characteristics were discussed based on transmission, DOS , and threshold behavior. From these results, it was demonstrated that the configuration of L-helical CLC sandwiched by polymer-dye doped R-helical PCLC layers is very attractive and efficient for lasing, suggesting that it can be a candidate of CW lasing. In section 6, we investigated the optical properties of an OHAS , composed of a nematic liquid crystal anisotropic layer sandwiched by two cholesteric liquid crystal layers possessing heterogeneous photonic bandgaps. We showed experimentally the electro-tunable optical diode (OD) performance (non-reciprocal transmission property) in transmittance and lasing when a half-wave phase retarder is incorporated as the anisotropic layer. In section 7, we concluded them.

This talk is part of the Optoelectronics Group series.

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